Combat agility requirements in the new generation of fighter aircraft have emphasized the need for controlled flight capability to increasingly high angles of attack. Such aircraft commonly employ pointed forebody shapes which characteristically experience abrupt and relatively large out-of-plane aerodynamic load when pitched to high angles. With degraded control effectiveness also encountered under such conditions, serious handling difficulties can result. Suppression or alleviation of the forebody side forces (and related yawings moment) thus has an important role in the current efforts to evolve aerodynamic configurations of improved controllability at extreme nose-high attitudes.
The out-of-plane force arising from asymmetric development of the leeward vortex wake of slender lifting bodies has attracted increasing attention in recent years by researchers in both government and industry. In this research, the two-dimensional impulsive flow analogy has provided useful conceptual and analytical framework, although its limitations are now increasingly being recognized. Recent experimental results have emphasized the influence of the vehicle nose shape on the outset and build up of side-force. This result is of particular significance to the engineering problem of side-force suppression by aerodynamic means. However, lacking comprehensive knowledge of the leeward flow-field impedes a rational approach.
Perhaps the best known among the side force alleviation devices for slender pointed bodies so far investigated are the nose-strakes. When suitably located near the body nose tip, strakes in many cases have successfully overcome the asymmetry problem, but sometimes at the expense of directional stability on aircraft configurations, as reported in NASA TN D-7716 (1974). Nose-strake performance appears to be overly dependent on the forebody configuration, and the development of effective strakes for a specific application seems to require much trial and error.
Detracting from the usefulness of nose-strakes, is the possibility of adverse interaction of strake vortices with downstream components such as air intakes, control surfaces, etc. The strakes must of necessity be mounted near the tip of the nose radome, where they are a potential source of disturbance in radar operation. Keeping in view such practical considerations, an alternative approach to the problem has led to the present invention, i.e., a new device for side-force alleviation.
Accordingly, it is an object of the present invention to provide apparatus for alleviation of side force encountered at high angles of attack by slender pointed forebodies.
It is another object of the present invention to provide apparatus for improving the high angle of attack flight characteristics of aerodynamic vehicles.
Another object of the present invention is the provision of surface changes to a slender pointed forebody for side force alleviation at high angle of attack.
A further object of the present invention is apparatus for cylindrical vortex wake suppression of slender pointed forebodies during high angle of attack maneuvers.
The foregoing and other objects are attained by the present invention by causing the normal fluid flow about a pointed slender body at angle of attack to separate at varying meridional locations along the body length.
It has been shown that the phenomenon of vortex shedding, which can lead to large oscillatory lateral forces on cylinders in cross flow, can be largely suppressed by means of certain types of separation trips attached to the cylinder. These trips force the boundary layers to separate at varying peripheral positions on different sections of the cylinder. The shed vorticity is thus rendered highly nonuniform along the cylinder, preventing its concentration into discrete two-dimensional cores. Consequently, asymmetrical vortex growth and shedding is replaced by random turbulent wake and the development of cross force on the cylinder suppressed.
In the present invention the leeside vortex pair resulting from meridional separation lines on either side of the exemplary lifting, pointed, slender body are disrupted by forcing separation to occur at varying meriodional locations along the forebody length by means of a suitable separation trip. In the preferred embodiment of the present invention this separation trip consists of a continuous helical ridge, running from the top meridian near the nose tip to a position near the bottom meridan of the forebody, symmetrically on either side of the pitch plane. The trajectory of the trip is such that it lies approximately normal to the boundary layer flow direction to effectively promote separation at large angles of attack while minimizing the drag penalty under normal flight conditions.